Electronic postage meters have been developed with circuitry for controlling various functions within the postage meter. Systems of this type have employed radiant energy devices to serve several functions such as for electrical isolation between circuits or for position indicators for moveable parts. Electrical isolation may be incorporated, for example, between the control circuits and accounting circuits of the meter which are housed in a secure tamper resistant enclosure. The electrical isolation enhances the security of the meter by electrically isolating the accounting circuit and its associated registers which store information representing funding for postage to be printed from other circuits which may be housed in separate less secure type enclosures. The radiant energy devices may also be employed as an optical coupler to provide information on the physical position of moveable components within the meter. The information is coupled to a microprocessor or other processing circuit to appropriately process the information.
Electronic postage meters employing radiant energy devices are disclosed in U.S. Pat. No. 3,978,457 entitled "MICROCOMPUTERIZED ELECTRONIC POSTAGE METER SYSTEMS" issued to Frank T. Check, Jr., Alton B. Eckert, Jr. and Joseph R. Warren and in pending U.S. patent application Ser. No. 63,359 for "ELECTRONIC POSTAGE METER HAVING NOISE-REJECTING INPUT/OUTPUT CHANNEL," filed Aug. 3, 1979 for Frank T. Check, Jr. and assigned to Pitney Bowes Inc. The radiant energy devices and their associated circuits disclosed in the patent and pending application operate satisfactorily for their intended purposes.
Since postage meters are utilized in various consumer locations where postage is to be printed, the meters are subject to a wide range of environmental conditions. Moreover, as registers in the postage meter store information representing funding of postage to be printed, improper operation or failure of the circuitry within the meter can result in a loss of funds to the user or to the postal authorities. Accordingly, reliable operation of the meter and its components with a minimum of servicing is extremely important, as is also the security of the meter against tampering. In circuits employing a radiant energy receiving member such as a phototransistor with the collector-emitter electrode, in series with a resistance, conflicting factors of voltage level and switching speed are encountered. The greater the resistance, the greater the voltage developed. However, with increases in resistance, the time required to discharge the inter-electrode capacitance increases. This increases the switching time of the device. The increase in switching time as a function of increases of voltage level can pose operational problems when the radiant energy device circuits are employed in conjunction with TTL (transistor transistor logic) circuits or data processing circuits which have both switching time and voltage level requirements for proper and accurate operation.
One TTL and microprocessing system requires a voltage level and a switching speed requirement for proper operation of 1.4 volts for a low (which can be designated to represent a logical zero) and greater than 3.6 volts for a high (which can be designated to represent a logical one). The transition from either a low to a high or a high to a low must occur within 5 microseconds.
The problem of selecting a trade-off between the voltage level and switching speed is compounded because of the many parameters associated with radiant energy devices which may vary from device to device. As a result, each circuit incorporating such devices may have to be individually adjusted to achieve a particular compromise of switching time and voltage level. One parameter for light emitting diodes (LED) is the amount of radiant energy emitted for a given current flowing through the device. This parameter is a function, in part, of the physical construction of the device. Another parameter is the mechanical alignment of the radiant energy emitting member and the radiant energy receiving member. The alignment controls the amount of radiant energy impinging on the radiant energy receiving member. A slight misalignment of a few degrees can significantly affect the amount of received energy which in turn controls the current flow through the radiant energy receiving member. The amount of current which will be generated for a given amount of incident radiant energy also varies from device to device and, in part, is a function of the device construction. The beta, or current gain, for typical phototransistors may vary from 200 to 20 between devices.
The above factors are also compounded by external factors in the postage meter environment. This includes contamination of the radiant energy device with dirt or grease from the mechanical components of the meter which coat the radiant energy emitting or radiant energy receiving member. The coating reduces the amount of effective radiant energy transmitted and/or received and thus reduces the operating level of the device. The interelectrode capacitance between the collector and the emitter electrodes of a phototransistor will also vary from device to device. Moreover, the interelectrode capacitance, the collector-emitter electrode leakage current, and the beta of the phototransistor can all "age," that is, vary over time. These parameters may vary with aging in a direction which increases the interelectrode capacitance and the leakage current and which reduces the beta. If care is not taken, the aged radiant energy device may cause the switching time and voltage levels to be outside of the required operating range of the circuits incorporating the device. All of the above variable parameters narrow the number of radiant energy devices which are suitable for use in a particular circuit and thereby increases the cost of the component by requiring the use of only devices within a limited range of parameters.
Typical optical circuits are disclosed in U.S. Pat. Nos. 3,886,351 for "OPTICAL-ELECTRONIC INTERFACE CIRCUIT"; 3,813,540 for "CIRCUIT FOR MEASURING AND EVALUATING OPTICAL RADIATION"; 3,772,514 for "ISOLATION AMPLIFIER" and 3,622,801 for "POST GENERATOR HAVING ADJUSTABLE THRESHOLD LEVEL." These circuits include optical members (radiant energy receiving members) connected to amplifiers. These circuits encounter the problems noted above.